Resilient materials, such as elastomers, have long been used in external prosthetic devices for the human body to cushion impact or shock loads. Because impact loads are necessarily and regularly encountered in walking, two common prosthetic devices that have often incorporated resilient materials are artificial feet and ankle joint prostheses for use with artificial feet. In early designs, an ankle joint prosthesis was typically a metallic pivot that included a plain (e.g., sleeve) bearing or a rolling element (e.g., ball) bearing. Resilient or elastomeric material was disposed both about the pivot to help limit its motion and in various portions of an associated artificial foot to cushion or absorb impact loads. Typical combinations of a cushioned artificial foot and an ankle joint prosthesis that incorporates a metal-on-metal pivot are described and illustrated in Ehle U.S. Pat. Nos. 487,697, Rowley 1,090,881, and Kaiser 2,183,076.
Later in the development of ankle joint prostheses for external use, resilient or elastomeric material came to be utilized in such prostheses for properties other than its ability to absorb or cushion impact loads. In Desoutter U.S. Pat. No. 1,911,440, for example, a tubular rubber bushing is secured between a pin and a metal sleeve that circumscribes the pin to form a pivot for an ankle joint prosthesis. The outer sleeve is connected to an artificial foot, while the pin is connected to an artificial lower leg. Articulation is permitted by torsional deflection of the bushing. Because of the resilience of the bushing material, the ankle joint prosthesis automatically returns to a preselected position after it is deflected. The prosthesis also does not require lubrication because the bushing separates the adjacent metal surfaces of the pin and the sleeve. Similar ankle joint prostheses that employ a tubular bushing or body of elastomer between an outer rigid sleeve and an inner pin or sleeve are described and illustrated in Burger et al U.S. Pat. No. 2,605,475 and Prahl U.S. Pat. No. 3,480,972.
A pivot or pivotable assembly that incorporates a relatively thin, tubular body of elastomer secured between a pin and a larger diameter sleeve is only capable of extensive rotational movement about a single axis. In a typical ankle joint prosthesis, such as the Desoutter and Prahl ankle joint prostheses, such an elastomeric pivot is oriented generally perpendicular to the longitudinal axis of the wearer's leg and transverse to the longitudinal axis of the wearer's artificial foot. In the orientation that has been described, the elastomeric pivot permits extensive flexion in the dorsal and planter directions. An elastomeric pivot so oriented, however, can only provide a limited degree of inversion and eversion of a foot about its longitudinal axis or a parallel axis and only a limited degree of internal and external rotation of the foot about the longitudinal axis of the lower leg. The motions other than flexion are all accommodated primarily through compression of the elastomeric bushing, which is relatively thin and cannot afford any significant degree of deflection. To overcome some of the motion limitations inherent in the ankle joint prostheses of the Desoutter and Prahl patents, the ankle joint prosthesis of the previously mentioned Burger et al patent incorporates two elastomeric pivots disposed at right angles to each other. The Burger et al ankle joint prosthesis thus can resiliently permit both extensive dorsal and plantar flexion and extensive inversion and eversion. Other external ankle joint prostheses attempt to provide the three types of movement afforded by a natural ankle joint through the use of relatively massive blocks of elastomer, rather than the tubular bushings discussed above. The blocks of elastomer may be specially shaped or contoured in order to provide appropriate stiffnesses or motion capabilities in the three critical rotational directions. Examples of external ankle joint prostheses that incorporate large blocks of elastomer are described and illustrated in Bennington et al U.S. Pat. No. 2,692,392 and Asbelle et al U.S. Pat. No. 3,982,280.
Although resilient materials, and particularly elastomeric materials, have for many years been suggested for use in external joint prostheses, the use of resilient or elastomeric materials in internal joint prostheses has only recently been proposed. The apparent delay in the appearance of proposals for the use of resilient or elastomeric materials internally of the human body is probably attributable in part to the lack of a physiologically inert elastomeric material that could safely be used in the body. Nonetheless, with the development of suitable elastomeric materials, such as Dow Corning Corporation's Silastic.RTM. silicone elastomer, a number of surgically implantable, elastomeric joint prostheses have been proposed, particularly for finger joints. The finger joint prostheses, in particular, tend to be entirely formed of elastomer or nearly so. Unfortunately, such designs require the elastomer to be bent of flexed extensively at some point to provide a pivot. The result is alternating tension and compression loading of the elastomer, which is detrimental to its long-term fatigue life. The use of notches in the elastomer to locate the pivot point further adds to the stresses in the elastomer. Examples of finger joint prostheses that are entirely formed of elastomer or nearly so are described and illustrated in Swanson U.S. Pat. Nos. 3,462,765, Niebauer et al 3,593,342, Lynch 3,681,786, and Swanson 3,875,594. Other than the finger joint prostheses mentioned above, relatively few implantable prostheses that employ resilient or elastomeric material have been identified. Nonetheless, the use of elastomeric material in an implantable hip joint prosthesis is suggested in Buechel et al U.S. Pat. No. 3,916,451, particularly FIG. 1, and in Bokros et al U.S. Pat. No. 3,707,006, particularly FIG. 5.
The ankle joint prostheses described in the previously mentioned patents to Desoutter, Burger et al and Prahl appear to represent the best presently known designs for use of the desirable properties of elastomeric material in a prosthesis that accommodates pivotal or rotational motion. Nonetheless, the elastomeric pivots that are incorporated in the ankle joint prostheses of these three patents do not make optimal use of elastomeric material within the space provided. In particular, the relatively thin, tubular bodies of elastomer in the ankle joint prostheses of Desoutter, Burger et al, and Prahl are subjected to relatively high, torsionally-induced strains which, over periods of extended use, will lead to failure of the elastomeric bodies. While the strains experienced by the elastomeric bodies of the patented ankle joint prostheses may not be detrimental in terms of a few hundred or even a few thousand articulations of the prostheses, the strains are critical when one considers several million articulations or deflections of the prostheses. Such numbers of articulations may easily be experienced during a year or two of normal use. In an ankle joint prosthesis that is used externally of the human body, replacement of the elastomeric elements of the prosthesis may merely represent additional expense and some inconvenience to the user. If such a joint prosthesis were implanted in the body of the user, on the other hand, failure of the elastomeric elements within one or two years would seriously limit the desirability of using such a prosthesis.